This application relates to instruments for measuring the total organic carbon content of water. More specifically, this application discloses and claims several improvements in the instruments disclosed and claimed in the assignee""s prior patents; these improvements can be used together, or, in some instances, as separate improvements.
The assignee of the present application, Anatel Corporation of Boulder, Colo., is the assignee of a number of U.S. (and corresponding foreign) patents relating to the measurement of the total organic carbon content (TOC) of water. Monitoring the TOC of water is highly relevant in a number of important industrial processes; in particular, the semiconductor and pharmaceutical industries both use ultrapure water in large quantities.
There are several basic methodologies known to the art for measuring TOC. Most involve oxidizing organic molecules in the water, using UV radiation (see commonly assigned U.S. Pat. No. 4,626,413) possibly together with a catalyst (see commonly-assigned U.S. Pat. No. 4,868,127), and/or oxidizing reagents such as perchlorates or persulfates, to drive the reaction, and measuring the CO2 thus produced. The CO2 can be measured in situ, typically by measuring the change in conductivity of the water sample itself (again, see commonly-assigned U.S. Pat. Nos. 4,626,413 and 4,868,127) or can be removed therefrom. For example, it is known to remove the CO2 by diffusion across a suitable membrane into a water sample of known conductivity and measure the change in conductivity of the latter; see Godec et al U.S. Pat. No. 5,132,094.
Another distinction drawn in the art is the method of determining the CO2 content based on a conductivity measurement. For example, it is possible to measure the change in conductivity of a static water sample over time, i.e., monitor the conductivity as the reaction proceeds, and determine that the reaction has been completed and thus determine the final TOC value by analysis of the rate of change of the conductivity. See commonly assigned U.S. Pat. Nos. 4,626,413 and 4,666,860. This process has the advantage of providing the most accurate possible measurement. However, this xe2x80x9cstatic samplexe2x80x9d process does take some time, on the order of several minutes, for the reaction to proceed to completion (or close enough to completion that the final value can be accurately inferred; see commonly assigned U.S. Pat. No. 4,868,127 at col. 18, lines 4-32.)
An alternative method of measuring TOC is known to the art, and is referred to as the xe2x80x9ccontinuous-flowxe2x80x9d technique. In a typical implementation of this process, the conductivity of a water stream is measured before it enters a cell in which it is exposed to a UV lamp, oxidizing organics in the stream. The stream then passes into a second conductivity cell. The difference in conductivity as measured in the two cells, that is, responsive to the partial oxidation that takes place in the UV cell, is indicative of the TOC in the sample. (See, e.g., Egozy U.S. Pat. Nos. 5,272,091, Chubachi 5,518,608). Alternatively, the UV exposure and the second conductivity measurement can take place in the same cell; see commonly-assigned U.S. Pat. No. 4,868,127 at col. 22, line 40xe2x80x94col. 23, line 38, and U.S. Pat. No. 5,047,212 at FIG. 18 and at col. 22. However, the TOC cannot be accurately be inferred from the partial oxidation performed in such a continuous-flow process, because the change in conductivity detected thereby is a function not only of the residence time of the sample in the UV cell, but also of the rate at which the TOC is oxidized thereby. As various organics are oxidized at substantially differing rates, the change in conductivity that occurs during partial oxidation is not itself sufficient to determine the actual TOC value. Therefore the ultimate accuracy of the xe2x80x9ccontinuous-flowxe2x80x9d technique is substantially limited.
However, the continuous-flow technique does have one particular advantage, namely, that it provides a relatively rapid response. Given typical flow rates and volumes, the continuous-flow technique can provide updated measurements on the order of every 30 seconds. This allows the continuous-flow technique to be useful in monitoring ongoing processes. For example, suppose an instrument implementing the continuous-flow technique is connected in-line to a process flow, and the change in conductivity measured between the two cells.(xe2x80x9cxcex94Cxe2x80x9d) is monitored continuously. A change in the TOC of the stream will be detected substantially instantaneously (again, in on the order of 30 seconds) as a change in xcex94C, and can be used to sound an alarm or the like.
It would be desirable to combine the rapid response characteristics of the continuous-flow instruments with the extreme accuracy of the static-sample instruments, and to do so is one object of the present invention.
The accuracy of the TOC measurement provided by any instrument is determined at least in part by the accuracy of the circuit used to measure the conductivity of the water sample involved, and the circuits used in the instruments sold by the assignee Anatel Corporation have evolved substantially over the last fifteen years. The original circuit is shown in U.S. Pat. No. 4,683,435; subsequent improvements are shown in U.S. Pat. No. 5,260,663 (the xe2x80x9c""663 patentxe2x80x9d hereinafter) and U.S. Pat. No. 5,334,940. A simplified circuit providing some of the advantages of that shown in the ""663 patent is shown in U.S. Pat. No. 5,677,190 (U.S. Pat. No. 5,677,190 was originally commonly-assigned with the present application, and is referred to herein as one of the xe2x80x9ccommonly-assignedxe2x80x9d patents.)
It is always desirable to improve such circuits, and to do so is one object of the present invention. Specifically, the response time of the instrument can be improved by increasing the intensity of the UV lamp used to drive the oxidation, but doing so increases the noise that the circuit must reject in order to function properly; it was therefore desirable to provide a more sophisticated circuit providing increased noise rejection. New components have also become available promising increased circuit performance, and to employ these to maximal advantage is another object of the present invention.
As set forth in the commonly-assigned patents mentioned above, the design of the cell in which the water sample is exposed to UV and its conductivity measured has also been refined over time. The original cell design is shown in U.S. Pat. No. 4,626,413; a first refinement is shown in U.S. Pat. Nos. 4,666,860, and 4,868,127, further refinements in U.S. Pat. No. 5,275,957, and a readily-manufacturable, relatively low-cost cell in U.S. Pat. No. 5,677,190.
It is desirable to incorporate the best features of each of these cells in an instrument that combines the advantages of the continuous-flow and static sample approaches to TOC measurement, and such is a further aspect of the invention.
Finally, as discussed in the commonly-assigned patents discussed above (all of which are incorporated herein by this reference) there are limits on the process of measuring the TOC of a water sample by oxidation of the TOC to CO2, and measuring the CO2 content by measuring the change in conductivity of the water sample. Specifically, the relation between TOC and resisitivity after oxidation is only linear where the conductivity of the water is sufficiently low (i.e., the resitivity is sufficiently high) that the CO2 thus generated in situ is dissociated as free ions in the water, and when the TOC is below a certain level. As a practical matter this limits such instruments to measurement of TOC in ultrapure water, having conductivity of at most 0.1 microSiemens/centimeter (xe2x80x9cxcexcS/cmxe2x80x9d) (equivalent to resistivity of at least 10 megohm-cm); at lower purity levels, the CO2 is partly dissolved and partly dissociated, necessitating a complicated compensation scheme to be employed. Similarly, typically the instruments are limited to measurement of TOC of up to 2000 ppb. It would be desirable to provide some means whereby these instruments could be used to accurately measure the CO2 content of water samples of a wider range of resisitivity and TOC contents, and to provide such capability is another object of the invention.
The present divisional application is directed to improvements in circuits and methods for measuring the conductivity of an aqueous sample. More specifically, disclosed herein is an improved circuit providing improved noise rejection by using a novel differential conductivity measuring circuit in conjunction with recently-developed xe2x80x9csigma-deltaxe2x80x9d analog to digital converters, which provides unprecedented accuracy.